Along with the development trend towards high density and high performance in the IC industry, a higher density and thinner semiconductor encapsulation technique is also required. Thus, there is an ever increasing demand on the performance and reliability of the encapsulation material. An anisotropic conductive adhesive composition has process features such as easy in processing, high throughput, and high yield, etc. Therefore, the use of an anisotropic conductive adhesive composition in an encapsulation for both the interconnection of IC and hermetic adhesion has become an ever more important selection. An anisotropic conductive adhesive composition mainly uses a control on the particle size and the content of fine conductive particles in combination with a processing technique to make the composition having conspicuous anisotropic properties in the electric conduction direction (thickness direction) and the insulation direction (planar direction). An anisotropic conductive adhesive composition usually comprises a resin as a binder and fine conductive particles. The binder mainly has the functions of preventing the ingress of moisture, adhesion and insulation, etc., and can be a thermoplastic or thermoset resin including a common styrene-butadiene copolymeric resin, a polyacrylate, an epoxy resin, a polyetherimide (PEI), a polyimide (PI), a polyamideimide (PAI), and a polyurethane (PU), etc. Common fine conductive particles include carbon black, metal particles, resin cores coated with a metal, and glass core coated with a metal, etc.
A typical anisotropic conductive adhesive composition is described in U.S. Pat. No. 5,162,087. As shown in FIGS. 2 and 3 therein, when two circuit boards 21 separated by an anisotropic conductive adhesive composition 5 are thermo-compressed, the circuits 20 on the two circuit boards 21 become electrically conductive in the thermo-compressing (thickness) direction, and insulating in the planar direction. This is due to metal-coated particles 3 contained in said anisotropic conductive adhesive composition 5 are clamped by the two circuits 20 and forming contacts, and the rest of the metal-coated particles 3 are dispersed in t he binder 1 and isolated by the binder 1. In order to avoid the occurrence of a short circuit in the planar direction of the circuits, a thin resin layer is formed on the surface of the particle by dry blending the metal-coated particles with a fine resin powder in this U.S. patent. However, the reliability of this dry blending depends on the grain size distribution of the fine resin powder. Furthermore, this thin resin layer is liable to be damaged during the blending process with the binder. Thus, it is difficult for said patent to successfully achieve the objectives of avoiding short circuit in the planar direction and the metal-coated particles being uniformly distributed in the binder.